1. Field of the Invention
The present invention relates to an apparatus for continuously cooling a metal strip, which is continuously travelling in the longitudinal direction thereof, so as to achieve a uniform temperature distribution in the width direction of the metal strip.
2. Related Art Statement
For example, continuous annealing of a metal strip such as a steel strip is carried out as follows: A metal strip continuously travelling in the longitudinal direction thereof is continuously heated to a prescribed temperature and soaked. Then, the metal strip thus heated and soaked, which is continuously travelling in the longitudinal direction thereof, is continuously cooled to a prescribed temperature at a prescribed cooling rate immediately or after slowly cooling to a prescribed temperature. Then, the metal strip thus cooled is continuously subjected to an overaging treatment or a tempering treatment.
For the purpose of cooling the metal strip in the above-mentioned continuous annealing treatment, the known methods include a water cooling, a gas cooling and a roll cooling. Among these cooling methods, the roll cooling has an advantage of permitting rapid cooling of the metal strip to any temperature. In this respect, the roll cooling is superior to the water cooling and the gas cooling.
As an apparatus for cooling a metal strip by the roll cooling, for example, Japanese Patent Publication No. 57-14,414 dated Mar. 24, 1982 discloses an apparatus for continuously cooling a metal strip, which comprises:
a plurality of cooling rolls, which are freely rotatable and in contact with a metal strip continuously travelling in the longitudinal direction thereof, for continuously cooling said metal strip, each of said plurality of cooling rolls having a length at least equal to the width of the metal strip, said plurality of cooling rolls having axes in parallel with each other, a cooling liquid flowing through the interior of each of said plurality of cooling rolls to continuously cool same, and at least one of said plurality of cooling rolls is displaceable toward said metal strip to control a contact area between the surface of said cooling roll and the surface of said metal strip (hereinafter referred to as the "prior art").
FIG. 41 is a descriptive view illustrating a typical apparatus for continuously cooling a metal strip for example, a steel strip based on the above-mentioned prior art. As shown in FIG. 41, a plurality of cooling rolls 2 comprising, for example, five rolls 2a to 2e, which are freely rotatable and in contact with a steel strip 1 continuously travelling in the longitudinal direction thereof, for continuously cooling the steel strip, are arranged with the axes thereof in parallel with each other, at prescribed intervals.
Each of the cooling rolls 2 has a length at least equal to the width of the steel strip 1, and a cooling liquid flows through the interior of the cooling roll 2 to continuously cool same. Each of the cooling rolls 2 is displaceable toward the steel strip 1 by a driving mechanism not shown, to control the contact area between the surface of the cooling roll 2 and the surface of the steel strip 1.
The steel strip 1 continuously travels in the arrow direction while coming into contact with each of the above-mentioned plurality of cooling rolls 2. In the meantime, the portion of the surface of the steel strip 1 in contact with the surface of each of the cooling rolls 2 is cooled. The contact area between the surface of the steel strip 1 and the surface of each of the cooling rolls 2 is controlled by causing each of the cooling rolls 2 to displace toward the steel strip 1. The steel strip 1 is thus continuously cooled to a prescribed temperature by the plurality of cooling rolls 2.
The above-mentioned prior art has the following problems: In order to continuously cool the steel strip 1 continuously travelling in the longitudinal direction thereof so as to achieve a uniform temperature distribution in the width direction thereof, it is necessary to bring the surface of the steel strip 1 and the surface of each of the plurality of cooling rolls 2 into close contact with each other uniformly in the width direction of the steel strip 1.
However, it is difficult to bring the surface of the steel strip 1 and the surface of the cooling rolls 2 into close contact with each other uniformly in the width direction of the steel strip 1 for the following reasons:
(1) The steel strip 1, when coming into contact with each of the plurality of cooling rolls 2, is bent into an arcuate shape by each of the plurality of cooling rolls 2, thus resulting in a saddle-shaped deformation of the steel strip 1 in the width direction thereof.
(2) Fluctuations of thickness in the width direction, a defective shape and non-uniform tension in the width direction exist in the steel strip 1.
(3) The contact with the high-temperature steel strip 1 causes occurrence of a roll crown resulting from the thermal deformation in each of the plurality of cooling rolls 2.
(4) The plurality of cooling rolls 2 are non-uniform in the surface roughness.
It therefore becomes particularly difficult for the surfaces of the both side edge portions in the width direction of the steel strip 1 to be in contact with the surface of each of the plurality of cooling rolls 2.
FIG. 42 is a graph illustrating a temperature distribution in the width direction of a steel strip 1, when continuously cooling the steel strip 1 under, for example, the following conditions by means of the cooling apparatus of the prior art as shown in FIG. 41:
TABLE 1 ______________________________________ (wt. %) Fe and incidental C Si Mn P S Sol. Al impurities ______________________________________ 0.015- Up to 0.10- Up to 0.008- 0.032- Balance 0.025 0.04 0.20 0.02 0.025 0.067 ______________________________________
In FIG. 42, the abscissa represents a distance from the side edge of the steel strip 1 toward the center in the width direction thereof, and the ordinate represents a temperature in the width direction of the steel strip 1.
As shown in FIG. 42, the temperature in the width direction of the steel strip 1 on the exit side of the cooling roll 2e is over the target temperature for cooling of 350.degree. C. in a portion within about 100 mm from the side edge of the steel strip 1, and is about 570.degree. C. at a position, for example, of 20 mm from the side edge of the steel strip 1, and about 350.degree. C. at a position of 100 mm from the side edge thereof. Thus, the temperature distribution in the width direction of the steel strip 1 on the exit side of the cooling roll 2e is non-uniform with a higher temperature on the side edge than at the center, the difference in temperature being approximately 220.degree. C. between the center and the side edge. This causes occurrence of a defective shape such as edge waves or heat buckles in the steel strip 1 after the roll cooling.
When the edge waves exist in the side edges of the steel strip 1, an abnormal travelling such as a zigzag motion occurs in the steel strip 1 continuously travelling in the longitudinal direction thereof in the next treatment process such as an overaging treatment process applied to the steel strip 1 after the roll cooling. In an extreme case, as a result, the steel strip 1 is broken, thus making it impossible to continue the operation. It therefore becomes necessary to reduce the travelling speed of the steel strip 1 after the roll cooling in the next treatment process, and this seriously impairs the operational efficiency. When the heat buckles are present in the steel strip 1, the steel strip is rejected as a defective product, thus reducing the product yield.
FIG. 43 is a graph illustrating an aging index (AI) in the width direction of the above-mentioned steel strip 1, when the steel strip 1 is subjected to an overaging treatment at a temperature of 350.degree. C. for two minutes, then to a temper rolling with a reduction of 1.5%, and then, to an aging treatment at a temperature of 100.degree. C. for 60 minutes. In FIG. 43, the abscissa represents a distance from the side edge of the steel strip 1 toward the center in the width direction thereof, and the ordinate represents an aging index (AI).
The higher yield point of the steel strip 1 resulting from the aging causes occurrence of a defective shape and a spring-back during the press-forming and deterioration of a yield point elongation and a buckling resistance of the steel strip 1. The extent of the occurrence of these defects differs in the width direction of the steel strip 1.
In the steel strip 1 having the above-mentioned dimensions and chemical composition, if the upper limit of the aging index (AI) up to which a yield point elongation does not occur during the press-forming is assumed to be, for example, 4 kgf/mm.sup.2, then, as is clear from FIG. 43, the portion from the side edge to about 90 mm of the steel strip 1 would have a high aging index of over 4 kgf/mm.sup.2. This leads to non-uniform mechanical properties of the steel strip 1 in the width direction thereof.
Under such circumstances, when continuously cooling a metal strip continuously travelling in the longitudinal direction thereof by means of at least one cooling roll, there is a strong demand for the development of an apparatus for continuously cooling a metal strip, which permits prevention of the occurrence of a defective shape such as edge waves or heat buckles in the metal strip and an abnormal travelling of the metal strip such as a zigzag motion in the next process, and makes available a high-quality metal strip having uniform mechanical properties in the width direction thereof through achievement of a uniform temperature distribution in the width direction of the metal strip, but such an apparatus has not as yet been proposed.